1. Field of the Invention
This invention relates to the field of threshold voltage control and, more particularly, to the control of the threshold voltage of a transistor with a feedback control system, to bias the transistor body voltage in such a way as to reduce the threshold voltage to a desired value.
2. Description of the Prior Art
In the last few years, the desire to lower the power supply voltages applied to integrated circuits, ICs, and thus reduce the power consumption while maintaining high reliability, has resulted in a significant decrease in the speed of the ICs. There have been attempts, in the prior art, to alleviate this problem by controlling the threshold value of the transistors. In the 1976 International Solid State Circuit Conference of IEEE, an article entitled xe2x80x9cA Threshold Voltage Controlling Circuit for Short Channel MOS Integrated Circuitsxe2x80x9d by Masaharu Kubo, Ryoachi Hori, Osamu Minato and Kikuji Sato was presented wherein a threshold controlling circuit which can automatically set a circuit threshold voltage free from the fluctuations in device fabrication processes, by adjusting the substrate voltage of a MOSIC chip with a negative feedback. Also, in the 1994 Custom Integrated Circuit Conference of IEEE, an article entitled xe2x80x9cSelf-Adjusting Threshold-Voltage Scheme (SATS) for Low-Voltage High-Speed Operationxe2x80x9d by Tsuguo Kobayashi and Takayasu Sakurai was presented wherein the threshold voltage fluctuations were reduced by self-substrate-biasing technique. A major difficulty with the techniques set fourth in these papers is that the transistor body is biased in the wrong direction or sense, e.g. negatively, with respect to ground, for n-channel transistors and thus requires an extra power supply and a more complex controller.
The present invention increases the speed of integrated circuits, particularly with small power supply voltages and thus maintains low power consumption while maintaining high reliability. The present invention biases the transistor body only positively, with respect to ground, for n-channel transistors and only negatively, with respect to the supply voltage, for p-channel transistors thus simplifying the prior art and eliminating the cost of an extra power supply.